A great deal of information is present in extracellular electrogram morphology about the physiology of conduction of the cardiac impulse (1-4). For this reason, the characteristics of electrograms in reentrant circuits are of interest and have been investigated because they might provide information on mechanisms of slow conduction and block that cause reentry. Particular attention has been paid to the occurrence, location and causes of low amplitude or long duration potentials, double potentials and fractionated electrograms (5-13). In addition, it has been reasoned that if electrogram characteristics in reentrant circuits are specific they might provide an easy and rapid means for locating those circuits without complete activation mapping (12-14). This might facilitate ablation of such circuits with surgical or catheter techniques (12-15). However, specific electrogram characteristics in reentrant circuits have not yet been identified.
The analysis of electrogram morphology in reentrant circuits has only been done for individual complexes. It has not been determined whether there are special and specific dynamic electrogram characteristics, that is, beat-to-beat changes in electrogram shape during the course of tachycardia. We have shown that large dynamic changes in electrogram morphology as quantified by an adaptive template matching technique (16) occur specifically at functional lines of conduction block that bound the central common pathway of figure of eight reentrant circuits (17), in a canine model of ventricular tachycardia. This characteristic provides information about possible mechanisms for block, as well as suggesting that this methodology might be applied to the localization of some ventricular reentrant circuits causing clinical tachycardia. A preliminary report has also been published in abstract form (18).